The invention relates to apparatus for examining eyes to detect the presence of absence of glaucoma therein. The apparatus includes means by which the level of the pressure of the aqueous humour which is inside the eye can be established and the disruption of the flow behaviour (which is brought about by a reduced functioning of the excretory ducts) can be detected. It is suitable for carrying out ophthalmodynamometric and ophthalmodynamographic examinations by means of the detection of the pulse pressure amplitudes of the aqueous humour.
Statistics show that glaucoma (aglaucopsia) affects 2 to 4% of adults more than 40 years old, but it is to be considered as a cause of blindness in 15 to 20% of cases. At the present time, glaucoma can still not be cured, but it can be discovered at an earlier stage and as a result the development of the disease can be remedied or delayed. An extensive examination of the population for glaucoma is therefore important, this being for the purpose of a successful treatment of the disease and for preserving vision as long as possible, particularly nowadays, because the average age is considerably extended.
The occurrence of glaucoma is signalled by a pathologically increased pressure of the aqueous humour. The accepted physiological upper limit of the aqueous humour pressure with a healthy eye is 21 mm Hg. A pressure of 22 to 26 mm Hg already indicates the suspicion of glaucoma, and a pressure value higher than 26 mm Hg means the certain pathological state of the eye. For those cases which are not certain, diagnosis can be achieved by varying the volume of the aqueous humour, which effect can be produced by means of a pressure exerted on the eye for a few minutes. By determining the amount of the aqueous humour which escapes or by noting the variation of the pressure, it is possible to detect an upsetting of the equilibrium between the production and the excretion, i.e. a disruption of the excretion process. According to the most recent test results, and also in the case when the physiological aqueous humour pressure is normal, this points to the presence of glaucoma.
The processes which serve for measuring the pressure of the aqueous humour are known as tonometry, and those processes which serve to detect a disruption in the excretion process or a variation in time required for the aqueous humour pressure to change are known as tonography.
It is possible with a live person directly to establish the pressure of the aqueous humour in the eye by opening the eye. However, apart from a few special operational cases, this is not advisable. A process has therefore been widely used, according to which the actual pressure of the aqueous humour is determined indirectly, according to the tension of the eye sheaths. However, this means that the results of the measurements are influenced in a given case by the physical properties of the cornea and of the sclera, which differ, depending on the person. Summarising, these properties are known as "rigidity".
The prior known tonometric processes can be divided into two groups: these are the impressionisitic tonometry and the aplanatic tonometry.
Using impression tonometry, the pressure of the aqueous humour is detected by the extent of penetration of a given load, which is exerted on the surface of a few square millimeters of the cornea. The degree of penetration is then made visible by means of an appliance and an indicating instrument, which is operated either by mechanical transmission or by means of a measurement signal converter.
With the aplanatic tonometry, the pressure of the aqueous humour is detected by a load which is necessary for achieving a specific surface. Used for observing the aplanation surface is a conventional optical device and the loading and the indication thereof are obtained by mechanical means. Using the aplanation process, the pressure of the aqueous humour can be established in more accurate manner and with less influence caused by the rigidity, but the impression process is also suitable for carrying out tonographic measurements.
Both processes have one particular disadvantage, which is of main significance with mass examinations, and this is that the measuring head which comes into contact with the eye is able to cause an epidemic and a disease of the sclera and cornea. For the purpose of avoiding this, the measuring head is to be cleaned or disinfected before each examination, but this is very comlicated and time-consuming and cannot always be carried out with success. The mass examinations accordingly have to be stopped (frequently for several months), more especially at the time of an epidemic. An additional disadvantage of the known devices consists in that the examination requires a local anaesthetic and the cornea can be easily damaged during the measurement.
These disadvantages can simply be overcome by a process, which is associated with the aplanatic tonometry, in which the measuring head does not come into contact with the eye, but the deformation of the eye is caused by a stream of air or gas which is blown on to the surface of the eye and which has a linearly increasing speed as a function of time. The instantaneously aplanated surface (which has a diameter of about 3.6 mm) is indicated by means of a light beam which is projected on to the eye and is reflected back from said eye. The pressure of the aqueous humour is determined by the period of time which meanwhile elapses. However, this method suffers from the disadvantage which consists in that the pressure of the aqueous humour fluctuates according with the pulse cycle, as a result of which the measurement is considerably influenced. The pressure fluctuation caused by the pulse amounts to 2-3 mm Hg. Such a pressure fluctuation masks the examination results, especially in boundary cases.
It is a primary object of the invention to eliminate said defects and disadvantages of the known devices. It is a further object of the invention to provide a comparatively simple apparatus which does not come into contact with the eye during the examination. It is yet a further object of the invention to provide more accurate measurement by the elimination of the pulse effect. It is yet a further object of the invention to provide apparatus which is equally suitable for tonographic and tonometric measurements, and for carrying out ophthalmodynamometric and ophthalmodynamographic examinations.